1. Field of the Invention
The present invention relates to a clock spring connector for use as an electric connection means in an air bag system included in a steering unit of an automobile.
2. Description of the prior Art
A clock spring connector comprises a fixed member, a movable member attached rotatively around the fixed member, and a flexible cable accommodated in a space formed between the fixed member and the movable member. When the clock spring connector is included in a steering unit, the fixed member is secured to the steering column portion and the movable member is connected to the steering wheel portion.
FIG. 4 is a cross sectional view showing a conventional example of a clock spring connector of the foregoing type which has been disclosed in U.S. Pat. No. 5,102,061. Referring to FIG. 4, reference numeral 1 represents a fixed member. The fixed member 1 comprises a disc-like bottom plate 2 having a central hole 2a, an outer cylinder portion 3 stood erect around the bottom plate 2, and a disc-like ceiling plate 4 having a central hole 4a and formed integrally with the top end of the outer cylinder portion 3. A movable member 5 having a shaft-insertion port 5a is rotatively mounted on the central portion of the fixed member 1. The movable member 5 is guided by the central holes 2a and 4a respectively formed in the bottom plate 2 and the ceiling plate 4. An annular space formed between the outer surface of the movable member 5 and the outer cylinder portion 3 accommodates a flexible cable 6. Although omitted from illustration, an end of the flexible cable 6 is passed through the outer cylinder portion 3 and protrudes outside of the fixed member 1. On the other hand, another end of the flexible cable 6 is protrudes outside through the top end of the movable member 5.
The thus-constituted clock spring connector is used in such a way that it is mounted on a steering unit of an automobile. At this time, the fixed member 1 is secured to the steering column portion in such a manner that a lead wire protruding from the fixed member 1 is connected to an air bag drive circuit or a horn circuit mounted on the car body portion. Furthermore, a steering shaft is inserted and fixed into the shaft-insertion hole 5a formed in the movable member 5. A lead wire protruding from the movable member 5 is connected to an inflater for the air bag or a horn switch or the like mounted on the steering wheel portion. Therefore, when the steering wheel (the steering shaft) is rotated clockwise or counterclockwise, its rotational force is transmitted to the movable member 5 and thus the movable member 5 is rotated. In accordance with the direction of the rotation of the movable member 5, the flexible cable 6 is wound around the movable member 5 or the same is rewound toward the outer cylinder portion 3. In either case, the electrical connection between the fixed member 1 and the movable member 5 is maintained by the flexible cable 6.
In the foregoing conventional clock spring connector, when the flexible cable 6 is wound or rewound, the two widthwise ends of the flexible cable 6 are, in a radial direction, moved between the bottom plate 2 and the ceiling plate 4 of the fixed member 1. Therefore, the movement of the flexible cable is not inhibited by the rotation of the movable member 5. Therefore, an advantage can be realized in that the flexible cable can smoothly be wound or rewound.
However, the arrangement that the rotational force of the steering wheel is transmitted to the movable member 5 through a joint portion between the steering shaft and the shaft-insertion hole 5a, that is, the arrangement that the force is transmitted at a position near the rotational center of the movable member 5 raises a problem in that the torque of the movable member 5 is reduced and therefore the force for winding or rewinding the flexible cable 6 becomes too weak. As a means for transmitting the rotational force of the steering wheel to the movable member 5, a method has been known in which a projection extending from the top end surface of the movable member 5 is received by a hub hole formed in the steering wheel. Since the foregoing conventional example has the arrangement that the two vertical ends of the movable member 5 are pivotally supported by the bottom plate 2 and the ceiling plate 4 through the central holes 2a and 4a, it is difficult to maintain a space in which the projection extends from the top end surface of the movable member 5. If the projection extends from the top end surface of the movable member 5, the torque of the movable member 5 cannot be enlarged satisfactorily because the distance from the rotational center of the movable member 5 to the projection is shorter than half of the radius of the central hole 4a of the ceiling plate 4.